This invention relates generally to devices for insufflating gas into a mass of molten metal such as steel, and more specifically, to an optical wear indicator that indicates when the device should be replaced.
The making of steel or other metals typically involves the introduction of gases into the ladle or vessel holding the molten metal to stir it. The gas is typically introduced into the ladle via a device called a stir plug. Such a stir plug may be mounted in the bottom or side of the vessel. Prior art stir plugs have taken numerous forms and constructions.
For example, one common type of stir plug comprises a solid, non-gas-permeable, conical refractory member disposed within a loose fitting metal or ceramic shell or canister. Such a "canistered" plug is commonly disposed within a seating block in the wall, e.g., the bottom, of the vessel holding the molten metal, and the purging gas is transported through the gap between the refractory cone and the metal canister into the molten metal.
Another common type of stir plug comprises a conical shaped member or plug formed of a porous refractory material through which the purging (stirring) gas is passed to produce fine gas bubbles to stir the molten metal. Thus, that type of plug utilizes the porosity of the material forming the plug to create a capillary system formed by the interstitial spaces between the porous material for carrying the stirring gas through the plug. Such plugs are commonly disposed within a seating block in the wall such as the bottom of the vessel.
Another type of uncanistered plug is the so-called directed porosity plug. That plug comprises a conical body of cast refractory material containing an array of fine (0.7 mm diameter) channels that run in a straight line from the bottom to the top of the plug. When these plugs are used, the gas is distributed very finely in the molten metal by means of the capillaries, but as it passes through the capillaries, it undergoes a very high degree of friction loss as a result of the turbulence which develops on the inside surfaces of the capillaries. The effects of this turbulence on the flow of gas decreases with increasing size of the capillary cross section. Thus, it is not possible to increase the diameter of the capillaries to any desired extent in order to minimize friction, since such action would enable the molten metal to penetrate too deeply into the capillaries and block them in the event that the flow of gas should cease.
Only a large number of capillaries can guarantee the very high gas flow rate frequently desired in a steel mill. From the production angle, however, this turns out to be very expensive. Thus, to reduce friction losses, it was found advantageous to form a conical stirring plug of a single or multi-part construction to provide plural identical joints (in the case of a multi-part construction) or slots or passageways (in the case of a single part construction) extending linearly from the bottom to the top of the plug. Such "jointed/slotted" plugs exhibit similar gas agitation properties as the capillary tube plugs, but with significantly smaller pressure losses.
Other types of prior art stir plugs are also disclosed in the following U.S. Pat. Nos.: 4,438,907 (Kimura et al.); 4,535,975 (Buhrmann et al.); 4,539,043 (Miyawaki et al.); 4,560,149 (Hoffgen); 4,647,020 (Leisch); 4,657,226 (Illemann et al.); 4,741,515 (Sharma et al.); 4,836,433 (Perry); 4,840,356 (Labate); 4,858,894 (Labate); 4,884,787 (Dotsch et al.); 4,898,369 (Perry); 4,899,992 (Thrower et al.) 4,905,971 (Rothfuss et al.); and 4,925,166 (Zimmermann).
Any refractory material, such as any of the foregoing stir plug devices, is subject to wear due to extreme operating conditions. As the stir plug is worn down, the longitudinal height of the plug decreases. Stir plugs must be replaced as soon as a certain critical minimal, residual or remnant height is reached. If the stir plug is permitted to erode too much before it is replaced, a burn-out of the ladle in which the stir plug is located might occur, which is not only dangerous, but costly to replace.
Stir plugs with various devices to facilitate the determination of the critical remnant height have been generally available but with certain tradeoffs. For example, electrical indicators are generally available, but they may be expensive to install and maintain, they require an external recording apparatus and are a possible source of disturbance in an already error sensitive system.
One type of electrical indicator is the device described in the U.S. Pat. No. 4,481,809 (LaBate) which utilizes several Hall effect transducers and circuitry to monitor the output thereof.
Another electrical indicator is that disclosed in German Patent No. DE 3,424,466 (Grabner) which utilizes two electrical wires within a probe. Both ends of the wires at the tip of the probe are separated. The wires consist of an alloy that melts at the critical temperature which indicates the critical wear height. The melting alloy closes the circuit and allows an electrical current to flow from the power source. Temperature indicators are also available, but suffer from similar drawbacks as do the electrical indicators. One such device is the one disclosed in German Patent No. DE 3,526,391 (Fischer) which utilizes a thermocouple located inside the body of a ceramic stir plug. The critical temperature inside the plug is measured to determine the critical wear height of the plug. One deficiency of this device however, is that in the event the temperature of the liquid metal destroys the thermocouple, the temperature can no longer be measured and therefore the plug may have to be prematurely replaced.
Other prior art devices measure thermal conductivity to provide an indication of the lifetime of a stir plug. For example, German Patent No. DE 3,833,503 (Rothfuss) discloses a valve configuration inside a ceramic gas stir plug. A low melting alloy keeps the gas flow valve control in the open position. Concurrent with the erosion of the stir plug, high temperatures will ultimately cause the alloy component to melt. This causes the gas flow valve to close, thus either reducing or eliminating gas flow. The reduced or discontinued gas flow indicates the stir plug wear.
Another device disclosed in German Patent No. DE 3,623,609 (Rothfuss) uses a gas flow restriction as a wear indicator. However, a gas flow restriction is not an unmistakable criterion, because a premature steel penetration of the gas passageways result in a low gas flow, thus causing a premature replacement of the stir plug.
German Patent No. DE 3,802,657 (Winkelmann) discloses a refractory wear indicator incorporated in a gas stir plug. This device is an optical indicator which makes use of the geometrical arrangement of the gas passageways situated inside the plug. A certain configuration of gas passageways inside the refractory cone separates the inner refractory portion from the outer one. The inner portion includes a round cross section at the tip of the stir cone. The round cross section changes into a square one at the bottom of the refractory cone. In use, when the metal has been tapped from the ladle and the stir plug is hooked up to a natural gas purging line, the natural gas escapes in a circle configuration from the plug. If the plug is worn down below the critical height, the configuration of the natural gas flames changes from round to square. This indicator system can only work as long as the gas passageways are free from steel. Steel penetration of the passageways prevents gas flow, thus rendering the passageways invisible.
U.S. Pat. No. 4,744,544 (LaBate et al.) describes a visual wear indicator for a metallurgical vessel that uses a metal rod which is inserted in the upper portion of a refractory body and extends inwardly of the surface of the body, at a length less than the known thickness of the refractory body. In this device, the metal rod and the refractory material therearound are elevated to the same temperature by the molten metal, but their light emission coefficients will be different whereupon the end of the rod will glow red hot while the surrounding refractory material exhibits a different color (appearance). Thus, one can readily determine if the cone has worn down beyond the length of the rod. While this wear indicator is generally suitable for its intended purposes, it still leaves much to be desired. In this regard, since the refractory plug and the steel rod of this wear indicator have different thermal expansion coefficients, elevated temperatures will result in different expansions of both materials, which action may crack the refractory body. Moreover, a faster wear of the refractory cone may occur, in part, caused by the drilling of the hole to accommodate the metal indicator. Further, the metal rod may be blown out of its hole by high gas pressure or it may prematurely melt away, thus effecting a premature plug exchange.
In copending U.S. patent application, Ser. No. 07/868,598, filed on Apr. 14, 1992, entitled Gas Stir Plug With Visual Wear Indicator, there is disclosed a gas stir plug device with a visual wear indicator which overcomes many of the disadvantages of the prior art. In particular, that device is of frusto-conical shape for introducing gas into a mass of molten metal and comprises a plug having an outer core formed of a first refractory material. A wear indicator in the form of a central core comprised of a second refractory material is located within a centrally located recess in the outer core adjacent the bottom end of the plug. The central core extends from the bottom end of the plug towards the top end of the plug and is of a predetermined height less than that of the outer core. The upper end of the central core when exposed by the erosion of the stirring plug provides a visual indication of when that plug should be replaced.
While the aforementioned invention overcomes many of the disadvantages of the prior art it still leaves something to be desired from the standpoints of simplicity of construction and cost.
Accordingly, a need exists for a visual wear indicator for a gas stir plug which overcomes the disadvantages of the prior art.